Method for setting modulation and coding scheme in wireless RAN system and apparatus supporting the same

ABSTRACT

A method and a transmitting station for transmitting data by using a multi-user multiple input multiple output (MU-MIMO) in a wireless local area network system are discussed. The method according to an embodiment includes transmitting a physical layer convergence procedure (PLCP) protocol data unit (PPDU) including a bundled interface field, a modulation and coding scheme (MCS) index field and a data field. The bundled interface field indicates a number of at least one spatial stream allocated to each of a plurality of receiving stations and a total number of spatial streams allocated to the plurality of receiving stations. The MCS index field indicates a same MCS index used for modulating and coding the at least one spatial stream. The data field is modulated and coded by an MCS scheme indicated by the MCS index field and transmitted via the plurality of spatial streams indicated by the bundled interface field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/686,590 filed on Apr. 14, 2015, which is a Continuation ofU.S. patent application Ser. No. 14/339,337 filed on Jul. 23, 2014 (nowU.S. Pat. No. 9,025,558 issued on May 5, 2015), which is a Continuationof U.S. patent application Ser. No. 13/392,837 filed on Feb. 27, 2012(now U.S. Pat. No. 8,824,400 issued on Sep. 2, 2014), which is filed asthe National Phase of PCT/KR2010/005465 filed on Aug. 18, 2010, whichclaims the benefit under 35 U.S.C. §119(e) to U.S. ProvisionalApplication No. 61/237,300 filed on Aug. 27, 2009, and under 35 U.S.C.§119(a) to Korean Patent Application No. 10-2010-0040590 filed on Apr.30, 2010, all of which are hereby expressly incorporated by referenceinto the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for setting a modulation and coding scheme(MCS) in a wireless local area network (WLAN) system supporting multipleinput multiple output (MIMO) and a wireless apparatus supporting themethod.

Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to 540 Mbps or higher, and is basedon a multiple input and multiple output (MIMO) technique which usesmultiple antennas in both a transmitter and a receiver to minimize atransmission error and to optimize a data rate.

The MIMO technique combines data streams which arrive with various timedifferences through various paths to effectively improve signalcapability of the receiver, and thus activates a function of a smartantenna. A single input single output (SISO) technique allows one systemto transmit and receive only one spatial stream at one time, whereas aMIMO technique allows transmission of multiple spatial streams. The MIMOtechnique can increase channel capacity in proportion to the number ofantennas without additional frequency allocation or transmit powerallocation. Channel capacity in a limited frequency resource isincreased by using multiple antennas at both ends, and a high data rateis guaranteed.

The IEEE 802.11n standard can perform data transmission by using achannel having four spatial streams and a 40 MHz bandwidth. In thiscase, an equal modulation (EQM) scheme in which all streams have thesame MCS level or an unequal modulation (UEQM) scheme in which eachstream has a different MCS level can be used.

Recently, a channel having a bandwidth of 80 MHz is used to provide athroughput of 1 Gbps or higher, and researches for supporting multi-user(MU) MIMO are actively ongoing to enable data transmission/receptionwith respect to an access point (AP) by utilizing the channelsimultaneously by several stations to effectively use the channel. Withthe use of the wider bandwidth and the support of the MU-MIMO, it isexpected to use more spatial streams to perform data transmission by astation and an AP having more transmission (TX)/reception (RX)interfaces.

SUMMARY OF THE INVENTION

The present invention provides a method for adaptively setting amodulation and coding scheme (MCS) level depending on a channelsituation in data transmission using a plurality of multiple streams anda wireless apparatus supporting the method.

The present invention also provides a method for decreasing overheadbased on setting of an MCS level in data transmission using a pluralityof multiple streams and for setting an MCS level having low complexity,and a wireless apparatus supporting the method.

According to an aspect of the present invention, a method for setting amodulation and coding scheme (MCS) performed by a transmitting stationwhich supports multi user-multiple input multiple output (MU-MIMO) isprovided. The method includes: dividing a plurality of transmission(TX)/reception (RX) interfaces of the transmitting station into at leastone bundled interface including at least one of the TX/RX interfaces;modulating a data stream to be transmitted through the TX/RX interfaceincluded in the at least one bundled interface by applying the MCS on abundled interface basis; and spatially multiplexing the modulated datastream and transmitting the multiplexed modulated data stream througheach of the plurality of TX/RX interfaces.

In the aforementioned aspect of the present invention, a modulationscheme may be individually determined/applied for each bundledinterface, and the same modulation scheme may be applied to a datastream transmitted through the TX/RX interface included in the bundledinterface.

In addition, bundled interface information used to allow a receivingstation to identify the TX/RX interface included in the bundledinterface and MCS information used to allow the receiving station toknow an MCS applied to the modulated data stream may be transmitted tothe receiving station together with the modulation data stream.

In addition, the bundled interface information and the MCS informationmay be transmitted by being included in a physical layer convergenceprocedure (PLCP) protocol data unit (PPDU), and the transmitting stationmay change the TX/RX interface included in the bundled interface on thePPDU basis and the MCS applied to the TX/RX bundled interface.

In addition, the bundled interface information may be a value indicatingthe number of TX/RX interfaces included per one bundled interface, andmay be included in a VHT-SIG of the PPDU as a subfield.

In addition, the bundled interface information may be a value indicatingan identifier of the TX/RX interface included in the bundled interface,and may be included in a VHT-SIG field of the PPDU as a subfield.

In addition, the MCS information may be a value indicating an indexnumber of an index set predetermined and stored in the receivingstation, and may be included in a VHT-SIG field of the PPDU as asubfield.

According to the present invention, data transmission using multiuser-multiple input multiple output (MU-MIMO) can be performed toincrease efficiency of radio resource utilization by adaptivelycontrolling a modulation and coding scheme (MCS) depending on a wirelessenvironment, a data priority, and an importance, and complexity can bedecreased in transmission performed by utilizing a plurality of spatialstreams.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of downlink data transmission using multiuser-multiple input multiple output (MU-MIMO).

FIG. 2 shows an example of a method for a bundled interface according toan embodiment of the present invention.

FIG. 3 shows an example of applying Hybrid Modulation (HyM) according toan embodiment of the present invention.

FIG. 4 shows an example of a physical layer convergence procedure (PLCP)protocol data unit (PPDU) frame format according to an exemplaryembodiment of the present invention.

FIG. 5 is a block diagram showing a wireless apparatus according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings.

FIG. 1 shows an example of downlink data transmission using multiuser-multiple input multiple output (MU-MIMO). In downlink MU-MIMO, anaccess point (AP) 100 transmits a training request (TRQ) frame 120 todata transmission target stations (STAs) after performing an enhanceddistributed channel access (EDCA) 110 based on the institute ofelectrical and electronics engineers (IEEE) 802.11 standard. In theexample of FIG. 1, the transmission target STAs are an STA_1 103 and anSTA_2 105. The TRQ frame 120 may include a transmission time and a listof target STAs for transmitting a data frame by using downlink MU-MIMO.In the example of FIG. 1, the list of target STAs includes the STA_1 103and the STA_2 105. The transmission time is a time required fortransmitting data by the AP 100 to the STA_1 103 and the STA_2 105.

If the STA which receives the TRQ frame is a target STA for receivingdata, a sounding physical layer convergence procedure (PLCP) protocoldata unit (PPDU) is transmitted to the AP. Herein, the sounding PPDU isa frame transmitted to allow an STA for receiving the sounding PPDU tobe able to estimate a channel state between a transmitting STA and areceiving STA. That is, upon receiving the TRQ frame 120, the STA_1 103and the STA_2 105 transmit a sounding PPDU 130 and a sounding PPDU 135to the AP 100.

If the STA which receives the TRQ frame is not the target STA forreceiving data, the STA configures a network allocation vector (NAV). Inthe example of FIG. 1, an STA_3 107 is not the target STA for receivingdata, and thus configures an NAV 138 and suspends a channel accessduring a transmission time period. The AP 100 receives the sounding PPDUand acquires channel estimation information for the STA. The AP 100, theSTA_1 103, and the STA_2 105 perform beam-forming, and the AP 100transmits a spatially-multiplexed data frame to the STA_1 103 and theSTA_2 105. Upon receiving the data frame, the STA_1 103 and the STA_2105 respectively transmit block acknowledgment (ACK) frames 150 and 155to the AP 100 in response to the received data frame.

In this case, the data frame is transmitted to the STA by using aspatial multiplexing scheme. The spatial multiplexing scheme is a schemefor transmitting a data stream through a multiple spatial channelprovided by using multiple antennas of the transmitting STA and thereceiving STA.

Meanwhile, a modulation process is performed to carry data, which iscoded by an encoder, on a carrier in data transmission. Variousmodulation schemes can be used such as amplitude shift keying (ASK),frequency shift keying (FSK), phase shift keying (PSK), quadratureamplitude modulation (QAM), or the like.

The PSK is less affected by noise in comparison with the ASK, and isless limited by a bandwidth in comparison with the FSK. When a signal isdivided by a shift of π/2 by varying a phase shift level, it allows twobits per phase shift. This is called 4-PSK or QPSK. Likewise, when aphase shift level is set to π/2, it allows three bits per phase shift,which is called 8PSK. When a PSK order increases by decreasing the phaseshift level, the number of bits that can be transmitted for each phaseshift increases, which implies the increase in a bit rate. However, whenthe phase shift level decreases to obtain a high bit rate, there is aproblem in that it is difficult for a receiving side to distinguish asmall phase change.

The QAM is a scheme of combining the ASK and the PSK. The QAM can bemodified variously. In theory, the QAM can be obtained by combining anymeasurable amplitude change and any measurable phase change. Accordingto the combination, various QAM modulation levels (i.e., 4-QAM, 8-QAM,16-QAM, 64-QAM, etc.) can be used in modulation. The QAM scheme isadvantageously less sensitive to noise in a sense that the meaning ofshift can be recovered from phase information even if a noise problemaccompanied by amplitude shift occurs when a specific phase and aspecific amplitude are associated according to a constellation design.

A transmitter determines which modulation scheme will be used byconsidering a received-signal environment of a receiver, power of thetransmitter, an importance of data to be transmitted, a data amount,etc. Examples of the modulation scheme include not only the ASK and theFSK but also binary PSK (BPSK), quadrature PSK (QPSK), 16-QAM, 64-QAM,etc.

When using the 64-QAM which is a higher order modulation scheme in asituation where the receiver experiences fading, it is expected thattransmission is not properly performed. Therefore, when a channelcondition is poor due to an influence of fading or the like,transmission is performed by using the QPSK having a low modulationorder. When the channel condition is good, the 64-QAM can be used forhigh-rate transmission.

In the PSK and QAM-type modulations, many symbols can be carriedconcurrently on a carrier when a modulation order is high. Therefore,although a more amount of information can be carried when using the16-QAM in comparison with the 4-QAM and when using the 64-QAM incomparison with the 16-QAM, more power is consumed to identify eachsymbol. Consuming the more power implies the increase in an inter-symboldistance. The increase in the inter-symbol distance implies that symbolscan be easily identified from one another in a probability sense. Whenusing the same power, high order modulation has a high error rate indata transmission.

A plurality of spatial streams can be subjected to coding, modulation,etc., and then be transmitted through multiple antennas. The pluralityof spatial streams transmitted through multiple antennas of thetransmitter can be received through multiple antennas of the receiver.

Two modulation schemes for spatial streams can be taken into accountwhen performing transmission/reception through multiple antennas. One isa scheme of applying one modulation scheme to all streams whenmodulating a plurality of data streams, and the other is a scheme ofdetermining a modulation scheme for each data stream. The former iscalled equal modulation (EQM), and the latter is called unequalmodulation (UEQM).

Since the EQM uses the same modulation scheme for all streams, there isan advantage in that the transmitter and the receiver can be implementedwith less complexity and a relatively small number of bits can be usedwhen a modulation scheme used by the transmitter is reported to thereceiver. However, when a plurality of streams are transmitted, themodulation scheme cannot be controlled by considering a differentenvironment for each channel. This implies in general that, when themodulation scheme is determined for data transmission, the modulationscheme and a modulation order are determined by the worst channel.Accordingly, low order modulation is applied even in case of a spatialstream transmitted through a relatively good wireless channel, which mayresult in waste of radio resources.

Unlike the EQM, the UEQM causes somewhat high complexity of thetransmitter and the receiver. In addition, a more number of bits areused when the transmitter reports the modulation scheme applied for eachstream to the receiver. When data is transmitted through N spatialstreams, if one of P modulation schemes is applied for each stream byusing the UEQM, the total number of possible cases is P^(N). Thisimplies that a bit value capable of indicating P^(N) cases has to beused to report the modulation scheme (in case of using an MCS table toindicate the modulation scheme, an index value thereof is in the rangeof 0 to P^(N)−1).

Advantageously, however, according to a condition of a channel throughwhich each spatial stream is transmitted, a spatial stream transmittedthrough a good channel is modulated by applying a relatively highmodulation order, and a stream transmitted through a poor channel ismodulated by applying a low modulation order, thereby being able toperform transmission optimized for the condition of the radio channel.This implies that limited radio resources can be effectively used.

The present invention proposes a new MCS method as a method for solvinga problem in that the number of MCS index sets sharply increases inproportion to the number of spatial streams in use and for effectivelyusing radio resources.

According to an embodiment of the present invention, transmission(TX)/reception (RX) interfaces of a transmitter and a receiver aremanaged by dividing the interfaces into several groups. This is called abundled interface in the present invention.

FIG. 2 shows an example of a method for a bundled interface according toan embodiment of the present invention.

In the example of FIG. 2, a transmitting STA supports 16 TX/RXinterfaces, and each receiving STA supports 8 TX/RX interfaces. Herein,the receiving STA and the transmitting STA are relative concepts, andcan be mutually changed to each other anytime according to a directionof data frame transmission. The same is applied hereinafter.

In this case, when using the bundled interface proposed in the presentinvention, the 16 TX/RX interfaces of the transmitting STA are bundledinto four groups, and thus it is regarded as having four TX/RXinterfaces. Likewise, the 8 TX/RX interfaces of the receiving STA arebundled into two groups, and thus it is regarded as having two TX/RXinterfaces. In this case, when UEQM is applied on a bundled interfacegroup basis, the number of MCS index sets can be decreased.

In 16×16 MIMO transmission, if respective spatial streams use differentMCSs, and a MCS value applicable to one spatial stream is 8, then atotal number of MCS index sets is 8¹⁶. When a bundled interface is usedto effectively support the UEQM in MIMO transmission which transmitsdata through a plurality of spatial streams, the number of MCS indexsets can be adaptively changed from 8¹⁶ to 8⁸, 8⁴, etc., depending on achannel condition or the like.

According to the embodiment of the present invention, the EQM is usedfor a spatial stream that constitutes one bundled interface, and theUEQM is used for each bundled interface. Such a modulation schemeproposed in the present invention is called a Hybrid Modulation (HyM)scheme. The HyM using the bundled interface effectively supports theUEQM in MU-MIMO while decreasing the number of MCS index sets, and canincrease a beam-forming gain.

FIG. 3 shows an example of applying HyM according to an embodiment ofthe present invention.

In the example of FIG. 3, a transmitting STA can transmit 16 spatialstreams through 16 TX/RX interfaces. When four interfaces are grouped asone bundle, the 16 TX/RX interfaces of the transmitting STA can bedivided into a first group 310, a second group 320, a third group 330,and a fourth group 340. In this case, for the four groups, a differentMCS can be applied to each group. An MCS0 is applied to a spatial streamtransmitted through a TX/RX interface of the first group. An MCS1 isapplied to a spatial stream transmitted through a TX/RX interface of thesecond group. An MCS2 is applied to a spatial stream transmitted througha TX/RX interface of the third group. An MCS3 is applied to a spatialstream transmitted through a TX/RX interface of the fourth group. Inthis case, the same MCS is applied to spatial streams transmittedthrough four TX/RX interfaces of each group. That is, although the UEQMwhich uses a different MCS for each group is applied on a group basis(i.e., a group of 4 TX/RX bundled interfaces), the EQM is applied interms of each of TX/RX interfaces in a group since the same MCS isapplied to the four TX/RX interfaces that constitute each group.

Although four TX/RX interfaces are considered as one bundle in theexample of FIG. 3, the number of TX/RX interfaces which are grouped asone bundle can be optionally controlled.

Upon receiving data modulated by using the HyM according to the presentinvention, HyM information needs to be reported to a receiver in orderfor the receiver to demodulate the received data. The HyM informationmay include information of bundled interfaces and/or informationindicating a modulation scheme applied to each bundled interface.

The information of bundled interfaces may include information indicating‘number of bundled interface (N_bi)’ information indicating the numberof TX/RX interfaces included in one bundled interface. For example, ifan N_bi value is set to 2 when 16 TX/RX interfaces are bundled, 8 groupsare generated by considering two TX/RX interfaces as one group, and ifthe N_bi value is set to 4, four groups are generated by consideringfour TX/RX interfaces as one group. Likewise, if the N_bi value is setto 8, two groups are generated by considering 8 TX/RX interfaces as onegroup. If the N_bi value is set to 1, this is a case of not using thebundled interfaces, and if it is set to 16, this may imply that 16 datastreams are modulated by using the EQM.

The information of bundled interfaces may include a number assigned to aTX/RX interface that constitutes the bundled interface with a differentformat for delivering the information of bundled interfaces.

Modulation scheme information applied to each bundled interface can betransmitted to the receiver as an MCS index number of an MCS index set.For this, the MCS index set can be stored in a management informationbase (MIB) or the like.

The information of bundled interfaces and modulation scheme informationapplied to each bundled interface can be transmitted through a separateframe for reporting this to the receiver or can be transmitted by beingincluded in a PPDU.

FIG. 4 shows an example of a PPDU frame format according to an exemplaryembodiment of the present invention.

A PPDU frame 400 includes a VHT SIG field 420 and a data field 450. Inaddition thereto, in order to coexist with a legacy STA based on theIEEE 802.11 a/b/g/n, the PPDU frame 400 may optionally further includean L-STF field, an L-LTF field, an L-SIG field, an HT-SIG field, anHT-SFT field, an HT-LTF field, or the like of the IEEE 802.11n standard.The data field 450 includes a physical (PHY) service data unit (PSDU).

Information of bundled interfaces and modulation scheme information maybe included in the VHT SIG field 420 of the PPDU as a subfield, or maybe included in the PPDU frame 400 as a separate individual field. Theexample of FIG. 4 is a case where the information is included in the VHTSIG field as a subfield. In the example of FIG. 4, the information ofbundled interfaces is included in a ‘number of bundled interface (N_bi)’subfield 423, which indicates the number of interfaces included as onebundle, as a subfield. Further, the modulation scheme information isincluded in a PPDU frame in an MCS index number subfield 425, whichindicates a number assigned to an MCS index set, as a subfield.

Upon receiving the PPDU, the receiving STA can know a modulation schemeapplied to each stream from the modulation scheme information and theinformation of bundled interfaces included in the PPDU, and can performdemodulation on the basis of the modulation scheme.

Since the information of bundled interfaces and the modulation schemeinformation are transmitted by being included in the PPDU, thetransmitting STA can set the information of bundled interfaces and themodulation scheme information differently on a PPDU basis. As a result,the modulation scheme can be determined/changed adaptively depending ona channel condition, a transmitted-data priority, or the like on thePPDU basis in data transmission.

As an example of data frame transmission according to the embodiment ofthe present invention, it is assumed that a transmitting STA supporting8 TX/RX interfaces perform data transmission. The transmitting STAtransmits a TRQ frame to transmission target STAs, and in responsethereto, receives sounding PPDU frames from the transmission targetSTAs. Upon receiving the sounding PPDU frame and obtaining channelmeasurement information with respect to the transmission target STA, thetransmitting STA performs beam-forming and transmits a PPDU frame. ThePPDU frame includes the N_bi subfield 423 indicating the number ofinterfaces included in one bundle and the MCS index number subfield 425as shown in the example of FIG. 4.

TABLE 1 MCS Modulation Index Stream 1 Stream 2 Stream 3 Stream 4 5316-QAM QPSK QPSK QPSK 54 16-QAM 16-QAM QPSK QPSK 55 16-QAM 16-QAM 16-QAMQPSK 56 64-QAM QPSK QPSK QPSK 57 64-QAM 16-QAM QPSK QPSK 58 64-QAM16-QAM 16-QAM QPSK 59 64-QAM 16-QAM 16-QAM 16-QAM 60 64-QAM 64-QAM QPSKQPSK 61 64-QAM 64-QAM 16-QAM QPSK 62 64-QAM 64-QAM 16-QAM 16-QAM

Table 1 shows an example of an MCS index set used in the IEEE 802.11nstandard. Although an index set based on the IEEE 802.11n standard isdescribed for example, this is for exemplary purposes only, and thusvarious MCS combinations can be newly defined for the embodiment of thepresent invention. In addition to modulation information of each stream,the index set may further include a coding rate, number of pilot valuesper OFDM symbol, number of coded bits per OFDM symbol, number of databits per OFDM symbol, total bits per subcarrier, number of binaryconvolutional code (BCC) encoders for the data field, etc.

It is assumed in the present example that, when the index set of Table 1is used, the transmitting STA transmits a PPDU frame by setting an N_bisubfield value to 2 and by setting an MCS index number subfield value to58.

Upon receiving the PPDU frame, receiving STAs can know that 8 spatialstreams grouped into 2 bundles are modulated from the N_bi subfieldvalue of 2, and can know that 64-QAM, 16-QAM, 16-QAM, and BPSK are usedas a modulation scheme from the index number 58. Accordingly, thereceiving STAs can perform demodulation by calculating an MCS of each ofa stream 1, a stream 2, a stream 3, a stream 4, a stream 5, a stream 6,a stream 7, and a stream 8 respectively by the use of 64-QAM, 64-QAM,16-QAM, 16-QAM, 16-QAM, 16-QAM, QPSK, and QPSK. This is because the UEQMis used in terms of each group of bundled interfaces and the EQM is usedwithin one bundled interface according to the HyM proposed by thepresent invention.

FIG. 5 is a block diagram showing a wireless apparatus according to anembodiment of the present invention. A wireless apparatus 500 may be anAP or non-AP STA.

The wireless apparatus 500 includes a processor 510, a memory 520, atransceiver 530, and N antennas 550-1, . . . , 550-N. The transceiver530 transmits/receives a radio signal, and implements a PHY layer ofIEEE 802.11. The transceiver 530 supports MIMO transmission through theN antennas 550-1 . . . 550-N. The processor 510 coupled to thetransceiver 530 implements a MAC layer of IEEE 802.11. When theprocessor 510 processes an operation of a transmitting STA among theaforementioned methods, the wireless apparatus 500 is the transmissionSTA. When the processor 510 processes an operation of a receiving STAamong the aforementioned methods, the wireless apparatus 500 is thereceiving STA. The processors 510 and/or the transceiver 530 may includean application-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. The memory 520 may includea read-only memory (ROM), a random access memory (RAM), a flash memory,a memory card, a storage medium, and/or other equivalent storagedevices. When the embodiment of the present invention is implemented insoftware, the aforementioned methods can be implemented with a module(i.e., process, function, etc.) for performing the aforementionedfunctions. The module may be stored in the memory 520 and may beperformed by the processor 510. The memory 520 may be located inside oroutside the processor 510, and may be coupled to the processor 510 byusing various well-known means.

The aforementioned embodiments include various exemplary aspects.Although all possible combinations for representing the various aspectscannot be described, it will be understood by those skilled in the artthat other combinations are also possible. Therefore, all replacements,modifications and changes should fall within the spirit and scope of theclaims of the present invention.

What is claimed is:
 1. A method of transmitting data by a transmittingstation by using a multi-user multiple input multiple output (MU-MIMO)in a wireless local area network system, the method comprising:generating a physical layer convergence procedure (PLCP) protocol dataunit (PPDU), the PPDU including a bundled interface field, a modulationand coding scheme (MCS) index field and a data field; and transmittingthe PPDU to a plurality of receiving stations, wherein the bundledinterface field indicates a number of at least one spatial streamallocated to each of the plurality of receiving stations and a totalnumber of a plurality of spatial streams allocated to the plurality ofreceiving stations, wherein the MCS index field indicates a same MCSindex used for modulating and coding the at least one spatial streamallocated to each of the plurality of receiving stations, and whereinthe data field is modulated and coded by an MCS scheme indicated by theMCS index field and transmitted via the plurality of spatial streamsindicated by the bundled interface field.
 2. The method of claim 1,wherein the transmitting station is an access point (AP).
 3. The methodof claim 1, wherein the PPDU further includes a legacy-short trainingfield (L-STF), a legacy-long training field (L-LTF) and a legacy-signalfield (L-SIG).
 4. The method of claim 1, wherein a maximum number of thetotal number of the plurality of spatial streams is
 8. 5. The method ofclaim 1, wherein the PPDU is transmitted in a bandwidth of 40 MHz or 80MHz.
 6. A device configured to transmit data by using a multi-usermultiple input multiple output (MU-MIMO) in a wireless local areanetwork system, the device comprising: a transceiver configured toreceive and transmit radio signals; and a processor operatively coupledwith the transceiver and configured to: generate a physical layerconvergence procedure (PLCP) protocol data unit (PPDU), the PPDUincluding a bundled interface field, a modulation and coding scheme(MCS) index field and a data field, and instruct the transceiver totransmit the PPDU to a plurality of receiving stations, wherein thebundled interface field indicates a number of at least one spatialstream allocated to each of the plurality of receiving stations and atotal number of a plurality of spatial streams allocated to theplurality of receiving stations, wherein the MCS index field indicates asame MCS index used for modulating and coding the at least one spatialstream allocated to each of the plurality of receiving stations, andwherein the data field is modulated and coded by an MCS scheme indicatedby the MCS index field and transmitted via the plurality of spatialstreams indicated by the bundled interface field.
 7. The device of claim6, wherein the device is an access point (AP).
 8. The device of claim 6,wherein the PPDU further includes a legacy-short training field (L-STF),a legacy-long training field (L-LTF) and a legacy-signal field (L-SIG).9. The device of claim 6, wherein a maximum number of the total numberof the plurality of spatial streams is
 8. 10. The device of claim 6,wherein the PPDU is transmitted in a bandwidth of 40 MHz or 80 MHz.